Field of the Invention
The present invention relates to a USB power supply apparatus.
Description of the Related Art
Battery-driven devices such as cellular phone terminals, tablet terminals, laptop computers, and portable audio players each include a chargeable secondary battery and a charger circuit that charges the secondary battery as built-in components. Known examples of such charger circuits include an arrangement that charges a secondary battery using a DC voltage (bus voltage VBUS) supplied from an external circuit via a USB (Universal Serial Bus) cable.
At present, as a charger circuit mounted on a mobile device, charger circuits that conform to a specification which is referred to as the “USB Battery Charging Specification” (which will be referred to as the “BC specification” hereafter) have become mainstream. There are several kinds of USB hosts or USB chargers (which will collectively be referred to as a “USB power supply apparatus” hereafter). As the kinds of USB power supply apparatuses that conform to revision 1.2 of the BC specification, SDP (Standard Downstream Port), DCP (Dedicated Charging Port), and CDP (Charging Downstream Port) have been defined. The current (current capacity) that can be provided by a USB power supply apparatus is determined according to the kind of USB power supply apparatus. Specifically, DCP and CDP are defined to provide a current capacity of 1500 mA. Also, SDP is defined to provide a current capacity of 100 mA, 500 mA, or 900 mA, according to the USB version.
As a next-generation secondary battery charging method using USB, a specification which is referred to as the “USB Power Delivery Specification” (which will be referred to as the “PD specification” hereafter) has been developed. The PD specification allows the available power to be dramatically increased up to a maximum of 100 W, as compared with the BC standard, which provides a power capacity of 7.5 W. Specifically, the PD specification allows a USB bus voltage that is higher than 5 V (specifically, 12 V or 20 V). Furthermore, the PD specification allows a charging current that is greater than that defined by the BC specification (specifically, the PD specification allows a charging current of 2 A, 3 A or 5 A).
FIG. 1 is a block diagram showing a USB host 900 that conforms to the USB-PD specification investigated by the present inventors. A receptacle (USB port) 908 configured as a cable plug opening is connected via a USB cable 202 to a USB device (slave device, which will be referred to as a “USB power receiving apparatus” hereafter) 200 that functions as a power supply destination. In the drawing, only a VBUS line and a GND line are shown.
A power supply circuit 902 generates a DC voltage VOUT to be supplied to the USB power receiving apparatus 200. A feedback circuit 904 feedback controls the power supply circuit 902 such that an output voltage VOUT of the power supply circuit 902 approaches a setting voltage VSET. With the USB-PD specification, the setting voltage VSET is selected from among 5 V, 12 V, and 20 V.
The output of the power supply circuit 902 and the receptacle 908 are connected to each other via a bus line 906. A switch SW1 and an inductor L1 are provided to the bus line 906 such that they are arranged in series. Furthermore, a smoothing output capacitor C1 is connected to the output of the power supply circuit 902. Moreover, an output capacitor C2 is connected in the vicinity of the receptacle 908.
A communication (COM) terminal of a controller 910 is coupled with the bus line 906 via a capacitor C3 for DC blocking. With the USB-PD specification, a modulation signal VMOD is superimposed on the bus voltage VBUS, which allows the USB host 900 and the USB power receiving apparatus 200 to communicate with each other via the bus line 906. The modulation signal VMOD thus superimposed is input to the COM terminal via the capacitor C3. The controller 910 determines the setting voltage VSET of the bus voltage VBUS based on the negotiation with the USB power receiving apparatus 200. Furthermore, the controller 910 notifies the USB power receiving apparatus 200 of the allowed value of the supply current (charging current) ISUPPLY.
The USB-PD specification provides a larger allowed supply current ISUPPLY than that provided by the conventional BC specification. Thus, the USB-PD specification requires strict overcurrent protection and strict overvoltage protection. Specifically, in order to provide such circuit protection, the switch SW1 is provided. The controller 910 has an overcurrent protection function of turning off the switch SW1 when the current value of the supply current ISUPPLY exceeds an overcurrent threshold value. Also, the controller 910 has an overvoltage protection function of turning off the switch SW1 when the output voltage VOUT of the power supply circuit 902 or otherwise the bus voltage VBUS exceeds an overvoltage threshold value.
As a result of investigating the USB host 900 shown in FIG. 1, the present inventor has come to recognize the following problem. That is to say, as the switch SW1, a power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is employed. However, a MOSFET having an on resistance of several mΩ is costly. Accordingly, in practice, in many cases, a MOSFET having an on resistance that exceeds several tens of mΩ is employed. In this case, a power supply path between the power supply circuit 902 and the receptacle 908 has an impedance on the order of 100 mΩ, which is estimated based on the inductor L1 and the impedance of the bus line 106 itself in addition to the on resistance of the MOSFET. Thus, the voltage drop VDROP is not negligible. Giving consideration to the voltage drop VDROP, the bus voltage VBUT is represented by the following Expression (1).VBUS=VOUT−VDROP   (1)
For example, when a supply current ISUPPLY of 5 A flows through a resistor of 100 mΩ, the voltage drop becomes 500 mA, which is a large value. That is to say, in a case in which the power supply circuit 902 generates a voltage VOUT of 5 V, the bus voltage VBUS output from the receptacle 908 becomes lower, and specifically, becomes 4.5 V. With the USB-PD specification, in a case in which the setting voltage VSET is set to 5 V, the lower limit value of the bus voltage VBUS is defined to be 4.75 V. Accordingly, as the supply current ISUPPLY becomes larger, it becomes difficult for such an arrangement to satisfy the specification. The same problem can occur in the USB Type-C specification.